When Earth Day observances first began in 1970, Cleveland had recently doused a pollutant-fueled fire on a section of the Cuyahoga River. Cities were often shrouded in thick blankets of smog. And large portions of Lake Erie were so fouled by industrial, farm, and sewage runoff that sections of the 241-mile-long lake were pronounced dead.
As an environmental issue, global warming was far down the list of concerns.
At the time, a small number of climate scientists noted a general cooling trend in Earth's climate. They even suggested Earth might be about to begin a millenniums-long slide into a new ice age.
It was a decidedly minority view, however. And it quickly faded as global average temperatures rose and as the burning of fossil fuels and deforestation pushed concentrations of greenhouse gases in Earth's atmosphere relentlessly higher.
Since that first Earth Day, the air over major cities is cleaner. Lake Erie is healthier. So is the Cuyahoga River, which groups in Cleveland would like to turn into a centerpiece of urban life. The improvements have come with "yes, but ..." as other environmental challenges have elbowed their way to the fore. But for the most part, tools are in place to deal with them.
So, how are we doing on global warming, now widely seen as the century's most pressing environmental issue?
For the most basic measure, let's turn to the atmosphere and the changes that human activities are imposing on the mix of gases it contains – primarily carbon dioxide (CO2).
Most climate scientists trace global warming to the relatively rapid buildup of atmospheric CO2 from the burning of fossil fuels long sequestered deep underground.
Though only 0.04 percent of all the gases in the troposphere, where weather happens, CO2 is second only to water vapor as the most abundant greenhouse gas. And where a water molecule may remain airborne for up to 10 days before returning to the surface as rain, a newly emitted molecule of CO2 can remain in the air for centuries.
"From the grossest physical indicator, we're not getting the job done as a planet," says Alden Meyer, director of the Union of Concerned Scientists office in Washington, of the steady rise in CO2 levels.
"But ... there's some good news in the fact that some countries are moving forward with domestic strategies" for curbing greenhouse-gas emissions, says Mr. Meyer, who closely tracks national and international efforts to deal with climate change. "Is it enough, fast enough? No. But are they better than business as usual? Yes."
Atmospheric CO2 concentrations have reached the highest level in at least 800,000 years, reaching 395 molecules of CO2 for every million molecules of all gases in the atmosphere – 395 parts per million – some 45 to 50 percent higher than preindustrial levels.
Over much of the past decade, researchers and climate negotiators had focused on 450 p.p.m. as a target for stabilizing CO2 levels in the atmosphere. Doing that, they thought, would yield roughly a 50-50 chance of holding the rise in global average temperatures to about 2 degrees Celsius (3.6 degrees Fahrenheit) by the end of the century – a target that international climate negotiators settled on after reviewing the potential effects of higher temperatures.
Five years ago, however, climate scientist James Hansen and colleagues published a paper that looked at environmental changes that global warming already was bringing and pointed to a 300 to 350 p.p.m. range as the target most likely to avoid the worst effects of global warming. If he's right, that means it could be a lot harder to keep the warming to within the 2 degree target.
Dr. Hansen retired from the National Aeronautics and Space Administration's Goddard Institute for Space Studies in New York this month to play a more activist role on climate issues.
Meanwhile, the Global Carbon Project reported in December that CO2 emissions are increasing at nearly the highest growth rates envisioned by the Intergovernmental Panel on Climate Change, a United Nations-sponsored scientific advisory body.
Within the Global Carbon Project's broad emissions numbers, however, one apparent hopeful spot has emerged, the project's international team of scientists says. Global CO2 emissions from land-use changes appear to have declined in absolute terms, as has their proportion of overall emissions.
Despite a sharp spike around 1997, when extensive peat and forest fires burned in Indonesia, CO2 emissions from land-use changes have eased from about 1.4 billion tons in 1990 to about 900 million tons in 2011. The team credits new efforts to combat deforestation – healthy forests lock up CO2 that plants take from the air – as well as replace trees that were felled. Where land-use changes accounted for 36 percent of global CO2 emissions in 1960 and 18 percent in 1990, the proportion stood at 9 percent in 2011.
Brazil, where deforestation has been rampant, has been one of the bright spots, especially over the past several years, notes Greg Asner, a researcher at the Carnegie Institution for Science's Department of Global Ecology at Stanford University in Palo Alto, Calif. Low commodity prices reduced the pressure to convert forests to farmland, he says, and Norway poured a lot of money into the Amazon Fund, designed to stave off deforestation. Brazil has also stepped up efforts to combat illegal logging.
These factors have combined to dramatically reduce deforestation in Brazil, which lost 27,777 square kilometers of tropical forest in 2004. Since then, fewer square kilometers have been lost nearly every year, reaching a preliminary figure of 4,656 square kilometers lost in 2012.
"That's still a lot," Dr. Asner says of the latest losses. "But it's real progress. We thought we were going to lose the system faster than it's being lost now."
Yet if the picture is brighter in Brazil, it's less so in Indonesia, which along with Brazil accounts for 60 percent or more of the globe's tropical forests.
"I've seen no evidence for a decrease" in deforestation, Asner says. "I've only seen increases."
In the US, CO2 emissions fell 4 percent between 2011 and 2012, notes Daniel Lashof, director of the climate and clean-air program at the Natural Resources Defense Council, an environmental-advocacy group in Washington. Since 2005, annual US emissions have fallen by 12 percent, he adds, even as the economy has grown.
Half the drop in emissions can be traced to reduced energy demand during the Great Recession, he continues. But the other half stems from improved energy efficiency and a switch to cleaner fuels – natural gas replacing coal, and growth in the contribution of renewable sources, such as wind energy.
Wind-generated electricity expanded its share of US electrical production from less than 0.5 percent in 2005 to 3.5 percent of US needs in 2012, even as improved efficiency in appliances, lighting, and other electrical devices flattened demand, Dr. Lashof says.
Higher fuel-economy standards for vehicles also are contributing to declining emissions in the US.
A key step toward reaching President Obama's goal of reducing US emissions to 17 percent below 2005 levels by 2020 would be to extend to existing coal-burning power plants the CO2 emissions standards the US Environmental Protection Agency is finalizing for new coal-fired plants.
Rising natural-gas prices may prompt utilities to burn more coal over the next year, which could lead to a 2 percent increase in US CO2 emissions. But Lashof says with the extension of CO2 emissions standards to existing power plants, in addition to initiatives the White House already has taken, the US would be on track to meet the president's 2020 emission-reduction target.
In China, where emissions grew by 9.9 percent in 2011, the government is talking about implementing a carbon tax, although the plan reportedly has been put off until next year. In the meantime, the country is working toward reducing the amount of energy consumed per unit of gross domestic product – the economy's carbon intensity – by 3.7 percent this year.
Each emissions giant has political hurdles to overcome in pursuit of its stated CO2 goals. For China, it's a mismatch between what Beijing wants and what provincial leaders will implement. For the US, it's congressional gridlock on the issue, notes the Union of Concerned Scientists' Meyer.
If warming of 1.5 to 2 degrees C above preindustrial levels is the goal, he says, it's still salvageable. But time is running out.